Oil shale rock represents a large hydrocarbon resource base from which energy can be recovered. The hydrocarbon constituents in shale rock are known as kerogen which is a mixture of hydrocarbons, in either dry or solidified form, produced by sedimentation of organic substances. The kerogen is contained within the interstices of the shale rock matrix and is generally about 15-25 weight percent of the rock. The kerogen is typically converted into shale oil by high temperature retorting of shale. Shale rock also contains other valuable inorganic mineral values such as uranium, copper, nickel, cobalt, molybdenum, vanadium, titanium, iron and aluminum. For example, western Colorado shale generally contains about 3 to about 6 wt. percent aluminum.
Because of the value of shale's kerogen and minerals, methods for extracting kerogen or the minerals are important. Thus, chemical treating methods other than retorting for concentrating the kerogen contained in oil shale have been disclosed. Chemical treatments of oil shale to recover mineral values contained in the shale have also been disclosed.
For example, Hunter, U.S. Pat. No. 3,238,038 discloses a method for recovering a precious metal such as gold and silver from a bituminous shale ore by treating the shale with an aqueous alkaline solution to increase solubility of the precious metal in an aqueous acid solution. The insoluble residue from the alkaline digestion step is then separated and contacted with an aqueous acid solution, which must contain sulfate, nitrate, chloride and iron ions. A solution containing the precious metal is then separated. Hunter does not disclose a technique for concentrating the kerogen in shale nor a technique for recovering minerals other than precious metals.
Huang, U.S. Pat. No. 4,069,296, discloses a method for recovering aluminum from raw or spent oil shale by contacting the shale with aqueous hydrochloric acid, removing the insoluble residues from the acid treating solution, and then precipitating hydrated aluminum hydroxide from the acid treating solution by contacting with an alkaline agent.
Haas, U.S. Pat. No. 3,859,413 extracts some of the alumina from a previously retorted dawsonite-bearing oil shale. Dawsonite is a sodium aluminum carbonate mineral, NaAlCO.sub.3 (OH).sub.2, having aluminum concentrations greater than 4 weight percent. Haas employs a dilute alkaline leaching solution containing 5 to 25 grams solid caustic per liter, at temperatures of 5.degree. to 40.degree. C. for short leach periods up to 1 minute.
Rust, et al., U.S. Pat. No. 4,130,627, discloses a method for recovering minerals from fly ash, not shale, by treating a fly ash water slurry with a sodium hydroxide solution at pH 11-14, recovering a solid residue, and then leaching the solid residue with hydrochloric or hydrofluoric acid to form dissolved chloride or fluoride salts of iron, aluminum, and other metals. The solution containing the dissolved salts is then electrolized to plate out at least iron and optionally one or more other metals, and to recover silicon dioxide by precipitation from the electrolysis solution. The electrolysis solution free of silicon dioxide is then treated to precipitate aluminum hydroxide by raising the pH of the electrolysis solution.
Drinker, U.S. Pat. No. 4,026,360, discloses an in situ method for producing a shale oil from a subterranean oil shale deposit. The method comprises contacting the shale deposit with a hot aqueous alkaline solution to form clay-like materials in the deposit which can swell when subsequently contacted with water. Fresh water relatively free of electrolytes is then injected into the deposit to form flow barriers and reduce permeability of the shale deposit. The flow barriers then direct the flow of subsequently injected fluids to produce shale oil. Drinker does not disclose a kerogen concentration method.
Fahlstrom, U.S. Pat. No. 4,176,042, discloses a method for recovering kerogen from bituminous sedimentary rock by crushing and finally dividing the rock in a plurality of grinding stages to a sufficient fineness to free the major part of the kerogen present in the rock. The kerogen can then be separated from the rock by emulsification, flotation or density-separation processes. To enable the rock to be finely divided more readily, the rock at least prior to the final one of the grinding stages is subjected to a leaching treatment to weaken mechanical bonds between minerals contained in the sedimentary rock. The leaching treatment is performed at temperatures above 50.degree. C. with acid solutions, "although basic solutions may also be used". Fahlstrom discloses chemical treating only as an aid to a physical separation of kerogen by grinding of the shale rock. Moreover, acid and basic solutions are disclosed as equivalent for kerogen concentration, but acid solutions are preferred by Fahlstrom.
International Patent Application No. WO 82/03484, Meyers et al., assigned to TRW, Inc., published Oct. 14, 1982, and titled "Extraction and Upgrading of Fossil Fuels Using Fused Caustic and Acid Solutions" describes a two-step treating method for raw shale ore. The oil shale is first contacted with a fused alkali at a temperature within the range of 250.degree.-400.degree. C. The solid residue obtained is then washed with a dilute aqueous acid solution to obtain a kerogen concentrate. In this method, the severity of the fused alkali treating conditions is such that no water is present in the treating solution nor is any water maintained within the shale being treated. In addition, there is no disclosure of mineral recovery carried out in conjunction with the kerogen concentration.
Applicants' copending application, Ser. No. 541,720, "Method for Treating Shale", filed Oct. 13, 1983, and incorporated by reference, discloses digesting shale with a caustic solution under conditions sufficient to substantially transform the minerals of the shale into extractable forms without substantial extraction of the minerals into the caustic treating solution. The resulting shale product is then converted into a kerogen concentrate by an acid digestion. However, additional mineral removal from this kerogen concentrate can be desirable.
There are also numerous disclosures involving treating shale which has been previously retorted, roasted, or pyrolyzed with acid or base solutions. Typical such disclosures are found in Van Nordstrum, U.S. Pat. No. 3,389,975; Hite, U.S. Pat. No. 3,481,695; Paul et al., U.S. Pat. No. 3,510,255; Van Nordstrum, U.S. Pat. No. 3,516,787; Templeton, U.S. Pat. No. 3,572,838; Ellington, U.S. Pat. No. 3,586,377; Hoss, U.S. Pat. No. 3,859,413. For example, Hite, U.S. Pat. No. 3,481,695 discloses a process in which oil shale is mixed with sodium carbonate or bicarbonate salt, the resulting mixture is retorted to drive off the shale oil, and the solid residue is then leached with water or dilute acid or base to extract some of the aluminum values from the solid residue.
Applicants are not, however, aware of any integrated method for concentrating kerogen contained in raw or spent shale in conjunction with recovering the mineral values contained within the shale. The disclosed methods also do not focus on obtaining a kerogen concentrate of reduced ash content. Therefore, one object of the invention is to obtain an improved kerogen concentrate of reduced mineral content as measured by its ash content. Another object is to recover minerals such as aluminum contained within the shale rock to improve the economics of the treating method. Other objects appear below.